Machining by means of a laser beam has become increasingly established as an alternative to conventional mechanical processes for machining materials, for example steel, such as milling, cutting or punching. The machining of workpieces by means of laser beams has many advantages. On the one hand ablation and wear of the tool can be completely avoided, since the energy of the laser beam is fully absorbed into the workpiece and no tool that is subject to wear has to be used. Accordingly, also workpieces that on account of their hardness or brittleness cannot be mechanically machined at all or only with a great deal of effort, can be efficiently machined. On the other hand by using the laser beam and specifying a contour that the laser beam has to follow, virtually any arbitrarily complex or fine structures can be projected onto the workpiece, which can then be ablated, cut to size or welded. Especially in the steel-processing or automobile industry sectors very hard materials often have to be fashioned into relatively complex shapes.
Thus, for example, in printed specification DE 10 2006 052 824 B4 a method is known for the laser beam cutting of a metallic component, in which the laser beam is moved in an oscillating manner during the tracing of the cutting contour. Detection of the radiation produced by the plasma and/or the reflected laser radiation does not take place, however.
From printed specification DE 102 61 667 A1 on the other hand a method and an apparatus for laser cutting are known, in which the plasma is continuously monitored and the result is related to the quality of the cutting procedure. If the quality of the workpiece is not satisfactory, the workpiece is rejected.
The result of the machining process can be influenced by a number of parameters. These parameters are for example the material of the workpiece, the energy introduced into the workpiece by the laser beam, the wavelength of the laser radiation or the focal length of the focused laser radiation. These factors are often determined beforehand by appropriate tests.
With conventional laser machining tools the speed of the process is mainly limited by the achievable speed of the positioning systems at which a contour-true machining is still possible, and in these cases speeds of 5-10 m/min can be achieved. At higher speeds either fine contours can no longer be followed or the positioning accuracy is too low. In addition, the process speed is also limited by the achievable intensities of the laser radiation, since at low intensities the contour has to be traced correspondingly slowly or repeatedly.
In order to obviate these disadvantages high-performance scanners were combined with high-output lasers. By using high-performance scanners a very fast beam deflection can take place, wherein at the same time high intensities of the laser radiation can be achieved by using high-output lasers.
With sufficiently high intensities it is possible that the material of the workpiece does not melt as usual and has to be expelled by a process gas, but instead sublimes almost completely, or melt that is possibly formed is also expelled sufficiently by the resultant vapour pressure without having to use process gas.
In order to permit the necessary intensities of the laser radiation, as a rule the radiation is focused by optical components so that the focal point is located on the surface or in the vicinity of the workpiece. High intensities are also achieved by an appropriately strong focussing of the laser beam. This has the result, however, that the diameter of the laser beam (the spot) has a correspondingly small extension in the work or focal area perpendicular to the axis of the laser beam. On the one hand with small spot diameters correspondingly finer contours can be traced on the workpiece. On the other hand the use of small spot diameters also brings disadvantages, however, in relation to the quality of the machining result. Thus, increased demands have to be placed on the positioning of the laser beam and therefore of the spot, whose diameter is in the region of 0.1 mm. If additional positioning systems are used apart from the scanner, an exact positioning can no longer be ensured. On account of the small spot diameter it may also happen that molten material will reoccupy already ablated regions. A cutting kerf for example can then close again.